The present invention relates to a monitoring method for quantity of bank, a forming method for a sheet and measuring method for sheet temperature thereof and apparatus therefor and, more particularly, to a monitoring method for quantity of bank and an apparatus therefor, a forming method for a sheet and an apparatus therefor, and further a measuring method and apparatus for sheet temperature preferable for putting the aforementioned monitoring method and sheet forming method into practice in a sheet forming wherein a plastic material such as plastics, rubber or the like, controlled at a desired temperature, is fed on an upstream side of a gap between a pair of forming rolls arrayed almost in parallel and controlled at a desired temperature each relative to a flat die, connected to an extruding machine and having a slit outflow portion, the plastic material being pressed and formed into a sheet on the forming rolls in the gap therebetween, while forming a bank or an accumulation of the plastic material on an upstream side of the gap of the forming rolls, and is carried out on a downstream side of the gap.
In the aforementioned sheet forming method, a plastic material at a desired temperature is fed on an upstream side of a gap between a pair of two forming roll which rotate in reverse directions and controlled at a desired temperature, from a flat die connected to an extruding machine, an accumulation of the plastic material, namely a bank, being formed on the upstream side of the gap, the plastic material forming the bank being drawn into the gap of the narrowest portion (hereinafter simply called "gap") during rotation of the forming rolls, thereby pressing and forming the plastic material and carrying it out on a downstream side of the gap as a sheet. One of essentials exerting an influence on the quality of the sheet obtained through the sheet forming method is to keep a quantity of bank or a height of the bank from the center of the gap at an appropriate value. A deficiency in the quantity of bank may invite a surface defect called "shrink mark", and on the contrary the case where the quantity of bank is excessive, a surface defect of wavy marks called "bank marks" which extends breadthwise of the sheet at intervals in the direction where the sheet runs, arises, and further the forming rolls are bent to deteriorate a thickness distribution in the running length of the transverse direction of the sheet. To produce the sheet having desired characteristics without causing the aforementioned surface defects and the deterioration of thickness distribution as mentioned above, it is necessary that the quantity of bank (height) be controlled more accurately.
Now, therefore, as disclosed in Japanese Patent Laid-Open No. 47955/1973 and Japanese Patent Laid-Open No. 28657/1978, there proposed is an apparatus wherein a light is applied to the bank from either the upstream side or sideways, with the quantity of bank being measured by reflection or passage of the light, and a supply of the plastic material from the flat die or a rotational speed of the forming rolls is controlled so as to adjust the quantity of bank to a desired value.
According to the aforementioned way of measuring the quantity of bank by the irradiation of a light, a maximum value of the quantity of bank only is obtainable from applying the light sideways, but a distribution in the cross (transverse) direction cannot be measured. Thus, when applying the light from the upstream side, the distribution in the cross direction or preferably of a whole area can be measured. However, an upstream side of the bank is narrowed like a valley by the forming rolls, the flat die being disposed as close as possible to the bank to close an upstream side of the valley-like portion for the most part, and practically, the bank is formed slightly in the inner part of the gap, therefore such measurement involves difficulty. Now, therefore, the aforementioned sheet forming technique owes much to experience of operators hitherto, and thus it is difficult to produce a quality sheet easily and accurately.